The detection range of search radar is limited by the curvature of the earth, making it difficult for ground based radar to detect very low level flying aircraft. One solution is to carry Airborne Early Warning (AEW) radar systems aboard search aircraft. However, existing AEW surveillance and interceptor radar systems in general have difficulty reliably detecting and therefore tracking low altitude targets at a large enough range to permit effective fire control responses. In spite of such difficulty, detection of these targets improves with better control over antenna characteristics, especially with respect to pointing angles, sidelobe suppression and operating frequencies.
The basis for directivity control in a phased array antenna system is radio frequency (RF) wave interference, and by providing a large number of equally spaced antenna elements fed from in phase currents, maximum directivity in a forward direction can be achieved. By modifying the phase angles of the electric signals representing a transmitted or received electromagnetic wave, a stationary antenna array can transmit and receive RF from different angles. With multiple antenna elements configured as an array, it is also possible, with a fixed amount of power, to greatly reinforce radiation in a desired direction, while suppressing radiation in undesired directions. However, a common problem with arrayed configurations is that a large number of sidelobes are apt to be present in the radiation field. These sidelobes are often undesirable, since they tend to consume power and reduce detection sensitivity. In general, the problem of undesirable sidelobes can be reduced by harmonizing or calibrating the antenna and the transmission system.
FIG. 1 depicts a conventional phased array radar apparatus disclosed in Japanese Patent Public Disclosure (Kokai) No. 63-167287. A phased array antenna 1 of elements 100-1 to 100-M are connected to respective transmission and receiving modules RM-1 to RM-M. A circulator 2 permits the use of each antenna element as a transmitter and a receiver element, referred to as duplex operation. The transmission side of the module RM is fed two transmission beam parameters, a pulse train F1 and a phase angle DP1 of the desired electromagnetic radiation field. Typically, the pulse train is formed in a system (not shown) that includes a pulse divider and an oscillator which combine to produce a predetermined pulse modulation consisting of a plurality of pulses. The pulses are coupled functionally to a transmitting pulse distributor 101 and to a phase shifter 5 located in modules RM-1 through RM-M. A transmitting beam controller 102 is operative, based on data derived from the received signal representing azimuth and distance, to determine phase shift parameters that would effectively direct the antenna's radiation in the desired direction. The beam controller 102 sends the result, labeled DP1 through DPM, to respective ones of the phase shifters 5 located in modules RM-1 through RM-M. Respective ones of the modules apply input signals to their respective power amplifiers to form a high energy output pulse to each of the respective antenna elements 100 through 100-M.
A typical transmit radiation pattern, prior to implementation of the invention, is depicted in FIG. 4, and has a main beam transmit lobe 408 and sidelobes 410. The ordinate axis is a measure of relative electromagnetic signal strength as a unit of power, and the abscissa is a measure of the radiation direction angle in degrees as measured from a line perpendicular to the plane of the antenna array 1. An objective of radar antenna design is to create a narrow beam width, low antenna dispersion, uniform field radiation and low sidelobe levels. Broadside arrays of Yagi antennas, characterized by a direction of maximum radiation perpendicular to the line or plane of the array, are typically used in these applications.
Most objects are capable of reflecting RF waves, but the degree to which RF power is reflected in the direction of the receiving antenna array 1 depends on the atmospheric conditions, weather, size and shape of a reflecting object, maximum radar range of the system, angle of the return and the characteristics of the RF transmit pattern. A transmitted radiation pattern as depicted in FIG. 4 has a main beam 408, which contributes to returns reflected along a line of zero degrees, and a series of sidelobes 410 focused at differing solid angles, which likewise contribute to returns reflected from their respective complementary directions.
Upon reception (FIG. 1), RF radiation from a reflected target is received, at the antenna array 1. For example, input supplied by the antenna element 100-1 is passed through circulator 2 to module RM-1 of receiver 3 where it is demodulated into an intermediate frequency and separated into its in-phase and quadrature components. Each antenna element 100-1 through 100-M sends its output to the respective RM module. The received signal components are converted from an analog signal to a pair of digital signals R1 representing its phase and amplitude. The digital signals R1 are sent to a distributor 400 where they are combined with digital signals R2 through RM, respectively, from the receiving modules RM-1 through RM-M. The distributor 400 outputs the set of digital signals R1 through RM, to a set of beam forming circuits 500-1 through 500-M, which are adapted to control as desired the phase and the amplitude of the reception data R1 through RM, and provide reception beams in the direction of the target. The degree to which an RF system is capable of controlling the accuracy of the phase and amplitude of the radiation pattern will determine the ability of the radar to detect difficult targets at ranges that permit the detection necessary for effective fire control responses.
The ability to distinguish a true target echo from noise by a phased array radar system during use of all the antenna elements 100-1 through 100-M is important to the success of AEW surveillance radar. Low on-aircraft antenna radiation patterns improve overall detection performance and also avoid inducing false alarms caused by undesirable sidelobes. The proximity of the electrically conductive skin of an aircraft is a contributing factor to the configuration of the sidelobes. During calibration of antenna systems, use may be made of advanced technologies in antennae calibration and adaptive processing algorithms to compensate for the aircraft reflections.
Current antenna calibration schemes generally require that the measurements and adjustments be made on the ground. In addition there are typical requirements for test signals to be designed into the radar systems or provided through external signal generators. This is a time consuming and costly procedure. As phased array radar applications are pushed to new limits, new and novel calibration techniques must follow.